# Full text of "Text Book Of Mechanical Engineering"

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```624                     Combination of Diagrams.

primarily the base line should be volumetric, so in representing
these diagrams to the same scale, the bases must be altered to
suit the volume of each cylinder respectively, and one pressure
scale be used throughout; we shall then see at a glance the
comparative work performed in each cylinder, and shall further
be able to judge how nearly the total diagram corresponds with
what should take place were the whole expansion to occur in one
cylinder under theoretically good conditions.

Strokes being equal, the area, or diameter squared, will
represent cylinder volume. The squares of the diameters are as
4 . 10*89 : 29*2. Taking clearance at \ cylinder volume for the
H. P., -fa for the I. P., and -^ for the L. P., they are represented
by -5, 1*1, and 2*65. In the large diagram, set up at MA a scale
of absolute pressures per sq. in., and measure volumes along OK
to any convenient scale. Thus the dotted rectangles CE, ZH,
and QU are obtained, in which the indicator diagrams are to be
inserted. Divide DE, GH, and JK, each into 10 parts, and erect
vertical lines, upon which pressures are to be placed, as taken from
corresponding lines on the diagrams w, x, and Y, being careful to
set them up to absolute scale; and the shaded curves are obtained.

Next mark point of cut-off B, from which to draw the satura-
tion curve. The latter being always shewn in terms of specific
volume (see Fig. 608), divide AB into 2*7 parts, or the volume of
one pound weight of steam at 165 Ibs. absolute pressure. The
method of division is shewn at ML : an inclined line is drawn and
2*7 divisions to any scale placed upon it; then parallel lines to ML
will divide the latter proportionately. The volume "41 cub. ft.
has thus been found, which being crossed by '35 Ibs. sq. in.
minus, gives the new origin for the curve BSR, to be drawn as a
hyperbola in the usual graphic manner (Fig. 620). A second
curve CTU may be traced by dividing AC into 27 parts andr
proceeding as before, the origin being then much nearer o.

By stepping the cut-off ML into the whole volume MK, the
number of total expansions 13*84 is found, the pound weight of
steam occupying at the end of the low-pressure stroke a volume
of 13*84 x 2 7 or 37*4 cub. ft. The shaded areas, then, further
represent the work done by one Ib. weight of steam, if the base
lines be specific volumes, and the pressures taken from the```